By Duncan Campbell | The government’s 42 days counterterrorism legislation came under fresh fire last night when a former director general of MI5 said the provision to hold suspects for six weeks without charge was excessive.

Dame Stella Rimington became the second former MI5 chief in three months to come out against the controversial measures. “It’s too much, quite frankly,” she said when asked about the 42-day limit during a talk in London about her new spy thriller novel. While she was uncertain what the right limit was, it was clear to her that 42 days was too long.

Rimington, who became the first woman director general of MI5 in 1992 and held the post until her retirement in 1996, also attacked the US-led “war on terror” for sending the wrong message and said that the treatment of prisoners at Guantánamo Bay had been very damaging. “If there was a moral high ground, Guantánamo has lost it,” she told an audience at the Crime Scene festival where she was launching her novel, Dead Line.

The 42-day plan faces rejection when it goes to the House of Lords after the summer recess, guaranteeing a bruising round of parliamentary “ping pong” in which the counterterrorism bill will travel back and forth between the Lords and Commons if the government insists on the plan.

Dominic Grieve, the shadow home secretary, who is strongly opposed to the measure, tells the Guardian today that he expects it to be rejected by peers. “I’m not a member of the House of Lords and I can’t tell their lordships what to do. But my impression is that the chances of the bill getting through in its current form in the House of Lords are low. And it’s very likely that the House of Lords will remove the proposal for 42 days altogether, which is what we would support.”

Two months ago, another former head of MI5, Lady Manningham-Buller, used her maiden speech in the Lords to attack the plans. She opposed them on a “practical basis as well as a principled one”.

A critical report from the all-party Lords constitution committee, which includes the former lord chief justice Lord Woolf and former Labour attorney general Lord Morris of Aberavon, said the draft bill risked undermining the independence of judges and the chances of a fair trial for suspects. Other notable opponents of the legislation include Sir Ken MacDonald, the director of public prosecutions, who sees no reason to extend beyond 28 days. Lord Goldsmith, former attorney general, warned that it would undermine fundamental British freedoms.

The former head of MI5 said that a “war on terror” gave the false impression that aspirations could be achieved by military action. In fact, she said, a combination of policing, intelligence gathering and diplomacy was far more likely to succeed than force of arms. Terrorism should be treated as a crime, she said, as it was in the UK. “With the ‘war on terror’, the United States has gone down the wrong line,” she added.

Asked about the introduction of ID cards, another key element of the government’s security strategy, she said that “they [ID cards] won’t make us safer.”